1. Field of the Invention
The present invention relates to an intermediate frequency circuit of a digital television tuner and, more particularly, to an intermediate frequency circuit of a digital television tuner capable of outputting an intermediate frequency signal of a digital television with a characteristic which is flat in an intermediate frequency band.
2. Related Art
FIG. 3 shows a conventional intermediate frequency circuit of a digital television tuner (hereinbelow, called intermediate frequency circuit). An intermediate frequency signal of a digital television outputted from a not-illustrated mixer (simply called an intermediate frequency signal) passes through an SAW filter 21 for band regulation and is supplied to (the base of) a transistor 22 for amplifying intermediate frequencies. A power source voltage is supplied to the collector of the transistor 22 via a choke inductor 23. An intermediate frequency signal outputted from the collector is supplied to a low-pass filter 25 via a DC blocking capacitor 24. The low-pass filter 25 is connected to an output terminal 26.
The low-pass filter 25 takes the form of a xcfx80 type low-pass filter having an input-side shunted capacitor 25a, a serial inductance device 25b, and an output-side shunted capacitor 25c. Its cut-off frequency is higher than the frequencies in the intermediate frequency band.
A nominal impedance is set so as to be a few hundreds xcexa9 (ohm) in relation to a circuit to be connected at the next stage. On the input side of the low-pass filter 25, a resistor 27 is connected in parallel to the choke inductor 23. On the output side as well, a resistor 28 is connected in parallel to the output-side shunted capacitor 25c. By both of the resistors, a predetermined nominal impedance is determined.
In the above configuration, the transmission characteristic in the output terminal 26 is determined mainly by the transmission characteristic of the SAW filter 21 and the amplification characteristic of the transistor 22.
For example, when the SAW filter 21 has a passband of 8 MHz and a center frequency of 36 MHz, the transmission characteristic in the output terminal 26 is as shown in FIG. 4 that the level in the intermediate frequency band (8 MHz) is inclined. The level is high at low frequencies and becomes low at high frequencies for the following reason. The higher the frequency becomes, the more the gain of the transistor 22 decreases and, further, the cut-off frequency of the low-pass filter 25 is higher than the intermediate frequency band.
As a result, a level difference occurs in digital television signals in the band, so that it is difficult to reproduce a video image with high fidelity.
There is a case such that static electricity of a high voltage generated in an assembling process of the intermediate frequency circuit of the invention is applied to the output terminal 26. Such static electricity passes through the low-pass filter 25 and is transmitted to the collector of the transistor 22. The static electricity vibrates in the passing process, so that it has a predetermined frequency component. Since the impedance of the DC blocking capacitor 24 in the frequency component is low and that of the choke inductor 23 connected to the collector of the transistor 22 is high, the static electricity is applied to the collector of the transistor 22 and a problem such that the transistor is destroyed occurs.
An object of the invention is, therefore, to provide an intermediate frequency circuit having a flat transmission characteristic in an intermediate frequency band, and in which a transistor is prevented from being destroyed by a shock wave of static electricity or the like applied to the output side.
As means for achieving the object, an intermediate frequency circuit of a digital television tuner of the invention has: a transistor for amplifying a television signal in an intermediate frequency band; a low-pass filter provided between a collector of the transistor and an output terminal; and a parallel tuning circuit provided between the collector of the transistor and the ground. Each of a cut-off frequency of the low-pass filter and a tuning frequency of the parallel tuning circuit is set to be equal to or higher than the intermediate frequency band.
The parallel tuning circuit and the low-pass filter are coupled to each other via a first capacitor, the parallel tuning circuit has at least an inductance device for supplying a power source voltage to the collector of the transistor, a terminal on the power source side of the inductance device is connected to the ground via a second capacitor, and a reactance of the first capacitor at a frequency equal to or lower than the intermediate frequency band is set to be larger than a reactance of the second capacitor.
The reactance of the first capacitor at a frequency in the intermediate frequency band is set to be smaller than a nominal impedance of the low-pass filter.